In a cellular wireless system, a service area is divided into a number of coverage zones generally referred to as cells. Each cell may be further subdivided into a number of sectors. Base stations may transmit information on downlink channels to wireless terminals in each of the sectors of the base station's cell simultaneously, using different frequencies in different sectors or, in some cases, reusing the same frequency bandwidth in each of the sectors. Wireless terminals may include a wide range of mobile devices including, e.g., cell phones and other mobile transmitters such as personal data assistants with wireless modems.
A problem with sectorized cellular communications systems is that transmissions by the base station into a first sector of a cell intended for a first wireless terminal may interfere with transmissions from the base station into a second sector, intended for a second wireless terminal. In the case of sectors of a cell, due to transmitter proximity, this interference can be significantly greater than in the case of a neighboring cell transmission, in which case the transmitter and the antenna of a neighboring base station is located in a different cell.
Inter-sector interference is particularly problematic for wireless terminals located in sector boundary regions, e.g., regions where the received signal strength levels from both sector base station transmissions, as measured at the wireless terminal, are nearly equal. Inter-sector interference may be reduced by restricting transmissions from being on the same bandwidth in an adjacent sector resulting in increased transmission reliability; however, this has the negative effect of reducing overall system capacity. Different types of information are often coded differently, e.g., using different block sizes and/or different amounts of error correction codes and, in some cases no error correction codes at all. Generally, where some form of signal coding is used, the larger the block size used in coding the greater the protection against burst errors where one or a few consecutive bits are lost, e.g., at one or more different locations in the coded block. Burst errors are common in the case of wireless systems and may be the result of unpredictable impulse noise occurring on one or more tones. Unfortunately, large block sizes are not well suited for all types of data. In the case of time critical control information for example, it may not be practical to code the control information in large blocks which could take a relatively long time to communicate over a wireless link before they could be decoded. Thus, small block sizes are often used for time critical data particularly where the data unit to be transmitted can be represented in relatively few bits. For example some control signals can be transmitted using a single or a few bits with these signals frequently being transmitted in relatively small blocks. In the case of transmitted blocks including multiple bits, e.g., 2 or 3 bits, repetition coding may be used, e.g., the data bit may be repeated. However, given the small number of bits, the information in a small block is still particularly prone to loss due to signal interference.
Some control signals are normally represented using several bits with such signals frequently being encoded as medium or intermediate size blocks. Such blocks normally include error correction coding bits or some other form of error protection. While error correction is supported, the medium size coding block may be more prone to errors due to bursty signal interference than blocks of larger size where error correction coding and data resequencing over the larger block size can provide better protection against short term interference bursts than may be possible in a medium sized coding block.
Information and/or control signals which are not particularly time critical may be grouped together to form larger blocks of data which are coded and transmitted as a unit, e.g., a large coded block. Large coded blocks are frequently used for non-time critical data and/or data that requires a large number of bits to be useful. Larger code blocks may include, e.g., hundreds or even many thousands of bits which are treated as a single block for error correction coding purposes.
From the above discussion, it can be appreciated that different types of information and/or different size blocks of data transmitted from the base station to a wireless terminal, can tolerate different levels of interference before impacting system operation and the reliability of the information being communicated.
In order to use bandwidth efficiently, it is generally desirable to reuse as much of the frequency spectrum in each sector as possible. Unfortunately, in the case of a sectorized cell, the greater the amount of frequency reused in each of the sectors the greater the risk of signal interference and the loss of data. As noted above, different types of data and different coded block sizes can often tolerate different amounts of interference before becoming unusable. Thus, while avoiding use of the same tones in adjacent sectors minimizes signal interference it may also lead to an unacceptable loss of bandwidth if applied to all coded blocks to be transmitted in a cell. Similarly, transmitting information on the same tone in each sector at the same time may result in an unacceptable error rate particularly with regard to coded blocks which are small in size, e.g., one or a few bits.
In view of the above discussion, it becomes apparent that there is a need for methods and apparatus that exploit the different levels of tolerable interference for different types of information, and thus provide multiple levels of trade offs between bandwidth and transmission reliability.